Spinal surgery procedures often require securing various implants to vertebrae of the spine. One such implant is the pedicle screw and its related components. Other components, such as rods, are then secured to individual pedicle screw implants in order to provide a support or fixation function between and among neighboring vertebrae. Both the rods and screws may have varying diameters and dimensions depending on patient and therapeutic needs. Due to the complex curvature and anatomy of the spine it is difficult to align the bone screw and rod holder assembly with the rod, particularly when spanning multiple segments. Traditionally, this required extensive bending and test fitting of rods to correctly approximate the rod holding portion of the pedicle screw. More recently, the polyaxial screw type has become widely available, which allows the rod receiving portion of the screw to pivot about the screw head. The pivoting head allows the rod holder to interface with the rod with only minimal rod contouring. These polyaxial screws are now the most common type of pedicle screw used today.
Current pedicle screw designs consist of several component parts including: 1) a threaded bone screw shaft which is anchored into the pedicle bone of the vertebrae 2) a rod holding member which is attached to the head of the bone screw to receive a rod for stabilization of the spine, and 3) a set screw which interfaces with the top of the rod holder to secure the rod into the holder to form a stable construct. These components are assembled into two primary design types: monoaxial and polyaxial screw systems. The monoaxial screws typically have a fixed angular relationship between the bone screw and rod holder. These screw designs dictate that the rod is held perpendicular to the direction of the bone screw. While these designs are strong and stable, they make it difficult to position the screw and rod properly and require a lot of rod bending to correctly approximate the rod holder and rod. Polyaxial designs allow the rod holding member to pivot on the bone screw head such that the rod holder can properly interface with a rod that is not perfectly perpendicular to the direction that the bone screw is inserted. After the set screw is used to secure the rod into the rod holder, the polyaxial design will lock into place and no longer allow the rod holder to pivot on the screw head.
Current designs suffer from some limitation in their functionality and the manufacturing requirements to encompass the myriad variations of particular surgical approaches. For example, many companies must offer both a monoaxial and polyaxial screw set, each with applications for specific surgical procedures. Similarly, current designs rely on rod holders that are designed for one specific rod diameter. Thus, if a manufacturer wants to offer a system that can use both 5.5 mm and 6.0 mm rods, they must manufacture two different sized rod holders, and they must manufacture both monoaxial and polyaxial variants of each size holder. This necessitates holding an inventory of parts in surgical centers, hospitals, and supply houses, many of which are rarely used.
Another limitation of current pedicle screw systems is a tendency for the set screw threads to cross-thread when approximated with the rod holding element. This may complicate assembly during operative procedures and increase procedure time to the detriment of the patient. Additionally, set screws that are cross-threaded or over-torqued can cause head splay, whereby the top portion of the rod holding element is deformed outward by the set screw. This causes poor rod stability and often necessitates the removal and replacement of the pedicle screw, again, increasing operative time, damage to the pedicle bone and the potential for surgical complications. Undetected head splay at the time of surgery could also lead to premature failure of the construct and necessitate additional surgical procedures at a later time. It is therefore advantageous to eliminate cross threading of the set screw and prevent head splay of the rod holding element to any extent possible.
An additional shortcoming in current systems is the inability to create a uniplanar pedicle screw configuration that is able to pivot in only one plane of motion rather than the combined motions of the polyaxial design. A uniplanar design is useful in complex spinal reconstructive cases where multiple segments are spanned with stabilizing rods and when lateral forces must be applied to a vertebrae to bring it into alignment with neighboring segments. With the traditional polyaxial designs, a lateral force applied to the rod holding element will cause the rod holder to pivot on the bone screw head rather than rotate the vertebral body into a desired alignment.
Hence there has been identified a need to provide an improved pedicle screw device as well as systems and methods of employing and utilizing pedicle screw assemblies. For example, it is desired that an improved pedicle screw assembly allow for a reduction of the necessary inventory of expensive medical components. The present invention addresses one or more of these long felt but unmet needs.